A wind turbine includes a rotor having multiple blades to transform wind energy into rotational torque that drives a generator, which is coupled to the rotor through a drive train and gearbox. The gearbox steps up the inherently low rotational speed of the rotor for the generator to efficiently convert the mechanical energy to electrical energy, which is fed to a utility grid.
Referring to FIG. 3, modern utility-grade wind turbines are generally operated in accordance with a design power curve wherein in a first region (Region I), the wind speed is too low to warrant turbine operation and the turbine blades are pitched to a full feather position corresponding to the pitch angle that produces minimum aerodynamic torque. At a wind speed sufficient for start-up (Vcut-in), the blades are pitched to a Region II nominal pitch angle wherein a maximum aerodynamic lift is generated to produce torque and turn the rotor. In Region II, the wind speed and generator torque are below “rated”, and blade pitch is generally held constant at an optimal angle to produce maximum aerodynamic torque. With an increase in wind speed in Region II, power captured by the wind turbine increases along with mechanical loads on the turbine structure and components.
At “rated” wind speed (Vrated), the wind turbine reaches its rated power in Region III of the design power curve. In this region, the wind turbine power is limited to rated power to maintain the machine loads within design limits. Generator torque is held constant and blade pitch is controlled to regulate turbine speed at rated speed. For safety and machine load considerations, the wind turbine is shut down for wind speeds in excess of a defined cut-out wind speed (Vcut-out).
The fatigue and extreme load limits over the design life of the wind turbine are, as expected, generated primarily at wind speeds in excess of Vrated, particularly at wind speed approaching Vcut-out. This wind speed is generally known for various wind turbine designs.
Prior attempts to reduce extreme and fatigue loads in Region III of the power curve included control profiles that reduced the turbine rated speed and generator torque. However, this resulted in a corresponding decrease in power, thereby adversely affecting the wind turbine's annual energy production (AEP). For example, the PCT publication WO 97/09531 describes a control methodology for reducing loads on a wind turbine wherein, at a defined wind speed, power is reduced as a function of wind speed by reducing the rotational speed of the turbine rotor.
Accordingly, an improved system and method are desired for reducing loads on a wind turbine at high wind speeds without sacrificing power and turbine AEP.